Words And Photos: Richard Holdener

From a simple math standpoint, making more horsepower requires nothing more than shifting torque production higher in the rev range. It is also possible to simply produce more torque in the same rpm, but with any given displacement, additional torque becomes more and more difficult.

Simply shifting torque production higher in the rev range becomes the only available method for improved power production. The reason this works is that horsepower and torque are mathematically related using the following formula: HP=TQxRPM/5252. Using this formula, we see that anything done to increase either torque (or the engine speed at which it occurs) will ultimately improve power.

An example works well to demonstrate this relationship. A typical 383 that produces 450 lb.-ft. of torque at 4,500 rpm, equates to 385 horsepower. If we produce the same 450 lb.-ft. at 5,000 rpm, the horsepower production jumps to 428 hp. On paper, making more power is easy, but the question now is how do you go about shifting that all-important torque curve in the real world?

Altering the power curve of any motor is as easy as swapping the appropriate performance parts. When it comes to shifting the torque curve, look no further than the big three power producers, namely the heads, cam, and intake manifold. To illustrate the effect each of these components had on the torque and power curve, we tested them individually on a 383 stroker Chevy supplied by Speedmaster. It is also important to note that the combined effect is every bit as important, as matching these components to produce power production in the desired rpm range provides optimum results.

The test mule featured a number of desirable components, including Speedmaster’s own 4-bolt block stuffed with a forged rotating assembly that included a stroker crank, H-beam rods, and flat-top pistons. The baseline test was run with a set of Speedmaster as-cast-aluminum performance cylinder heads, a COMP 218/224 hydraulic roller cam, and Speedmaster dual-plane, Eliminator intake manifold. This represented a solid street 383 that (as testing demonstrated) produced an excellent combination of idle quality, drivability, and a healthy torque curve.

In addition to the as-cast heads, 218/224 cam and dual-plane intake, the 383 was configured for dyno use with a 950 HP Holley carb, MSD distributor, and 1 3/4-inch dyno headers. All testing was run on 91-octane pump gas using Lucas 5W-30 synthetic oil.

Run in this baseline trim, the 383 produced peak numbers of 388 hp at 5,200 rpm and 453 lb.-ft. of torque at 3,800 rpm. The first upgrade in our torque-shift sequence was to replace the as-cast cylinder heads with a set of CNC-ported heads from Speedmaster. It should be noted that had we elected to test the three components (heads, cam, and intake) in a different order, the power gains offered by each would be different, though the combined effect would still be the same.

After replacing the as-cast heads with the CNC-ported heads, the peak numbers jumped to 426 hp at 5,700 rpm and 459 lb.-ft. of torque at 3,900 rpm. Note that the head swap raised the engine speed where the motor made peak power from 5,200 rpm to 5,700 rpm, but shifted the torque peak by just 100 rpm (from 3,800 rpm to 3,900 rpm). Equipped with the ported heads, the 383 lost power slightly below 3,400 rpm, but the next upgrade would offer much more of a shift.

The peak torque shifted by 400 rpm, from 3,900 rpm with the 218 cam to 4,300 rpm with the larger 248 cam, but the peak output actually dropped by 10 lb.-ft., from 459 lb.-ft. to 449 lb.-ft.. In addition to dropping the peak torque output by 10 lb.-ft., the power gains experienced above 5,000 rpm (see graph 2) were accompanied by equally significant torque losses below that point. The extra 44 hp at 6,000 rpm cost 44 lb.-ft. of torque at 3,200 rpm. Much like our final intake test, cam choice comes down to where you want your power production.

The final test run on the 383 was to compare the dual-plane to a single-plane intake. Having shifted the torque curve with the CNC-ported heads and wilder 248 cam, we suspected the 383 might benefit from a single-plane intake designed to further enhance high-rpm power production. As illustrated by the results, the shift offered by the intake swap traded power production much like the cam.

Replacing the dual-plane Eliminator intake with the single-plane Shoot-Out intake (both from Speedmaster) increased power production by 20 hp, bringing the peak number to 486 hp at a slightly higher 6,400 rpm. Peak torque actually dropped by 5 lb.-ft. (from 449 lb.-ft. to 444 lb.-ft.) and shifted by 400 rpm (from 4,300 rpm to 4,700 rpm).

When we finally called it quits, the combination of the heads, cam, and intake increased peak engine speed by 1,200 rpm. Equipped with the new components, peak torque actually dropped by 9 lb.-ft., but occurred 900 rpm later in the rev range. Though we lost a little torque, this Power Shift added nearly 100 additional horsepower to the test motor.

Sources

COMP Cams
compcams.com

Holley/Hooker
holley.com

Lucas Oil
lucasoil.com

MSD
msdignition.com

Speedmaster
speedmaster.com